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JUNE - JULY

2013

SUBMITTED BY:

SHASHANK JAIN

2ND

YEAR B.TECH

MECHANICAL ENGINEERING

ROLL NO. 06713103611GPMCE

Summer Training Report

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ACKNOWLEDGEMENT

I would like to thank N.T.PC. BADARPUR for providing me a golden

opportunity to work with them . The support and the environment

provided to me during my project was more than what anyone would

have expected.

I am very grateful to Mr. MAN MOHAN SINGH(DY. MANAGER) who

granted me the opportunity of working as a summer trainee at

mechanical Division.

I would also like to thanks Mrs RACHNA BHAL (H.R.) , Mr. G.DSHARMA(TRAINING COORDINATOR) and my instructors of

B.M.D.,P.A.M., T.M.D. and divisions without them I would not be able

to perform such a delightful job.

And at last I would like to thanks all the people involve in the training

who helped me in accomplishing it in such a wonderful way.

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TRAINING AT BTPS

I was appointed to do 6 week training at this esteemed organization from 24th

June to 3rd

August, 2013. I was assigned to visit various division of the plant,

which were:

Boiler Maintenance Department (BMD I/II/III) Plant Auxiliary Maintenance (PAM) Turbine Maintenance Department (TAM) Coal Handling Department (CHD/NCHP)

These 6 weeks training was a very educational adventure for me. It was

really amazing to see the plant by yourself and learn how electricity, which is

one of our daily requirements of life, is produced.

This report has been made by my experience at BTPS. The material in

this report has been gathered from my textbook, senior student reports and

trainers manuals and power journals provided by training department. The

specification and principles are as learned by me from the employees of each

division of BTPS.

SHASHANK JAIN

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INDEX

ABOUT NTPC ABOUT BTPS BASIC STEPS OF ELECTRICITY GENERATION RANKINE CYCLE

BOILER MAINTENANCE DEPARTMENT

PLANT AUXILIARY MAINTENANCE TURBINE MAINTENANCE DEPARTMENT MAINTENANCE PLANNING DEPARTMENT COAL HANDLING DEPARTMENT

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ABOUT NTPC

NTPC Limited is the largest thermal power generating company of India, PublicSector Company. It was incorporated in the year 1975 to accelerate power

development in the country as a wholly owned company of the Government of

India. At present, Government of India holds 89.5% of the total equity shares of

the company and the balance 10.5% is held by FIIs, Domestic Banks, Public

and others. Within a span of 31 years, NTPC has emerged as a truly national

power company, with power generating facilities in all the major regions of the

country.

NTPC's core business is engineering, construction and operation of power

generating plants and providing consultancy to power utilities in India and

abroad.

The total installed capacity of the company is 31134 MW (including JVs) with

15 coal based and 7 gas based stations, located across the country. In addition

under JVs, 3 stations are coal based & another station uses naphtha/LNG as

fuel. By 2017, the power generation portfolio is expected to have a diversified

fuel mix with coal based capacity of around 53000 MW, 10000 MW through

gas, 9000 MW through Hydro generation, about 2000 MW from nuclear

sources and around 1000 MW from Renewable Energy Sources (RES). NTPC

has adopted a multi-pronged growth strategy which includes capacity addition

through green field projects, expansion of existing stations, joint ventures,

subsidiaries and takeover of stations.

NTPC has been operating its plants at high efficiency levels. Although the

company has 18.79% of the total national capacity it contributes 28.60% of total

power generation due to its focus on high efficiency. NTPCs share at 31 Mar

2001 of the total installed capacity of the country was 24.51% and it generated

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29.68% of the power of the country in 2008-09. Every fourth home in India is

lit by NTPC. 170.88BU of electricity was produced by its stations in the

financial year 2005-2006. The Net Profit after Tax on March 31, 2006 was INR

58,202 million. Net Profit after Tax for the quarter ended June 30, 2006 was

INR 15528 million, which is 18.65% more than for the same quarter in the

previous financial year. 2005).

NTPC has set new benchmarks for the power industry both in the area of power

plant construction and operations. Its providing power at the cheapest average

tariff in the country. NTPC is committed to the environment, generating power

at minimal environmental cost and preserving the ecology in the vicinity of the

plants. NTPC has undertaken massive a forestation in the vicinity of its plants.

Plantations have increased forest area and reduced barren land. The massive a

forestation by NTPC in and around its Ramagundam Power station (2600 MW)

have contributed reducing the temperature in the areas by about 3c. NTPC

has also taken proactive steps for ash utilization. In 1991, it set up Ash

Utilizationivision

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Technological Initiatives

Introduction of steam generators (boilers) of the size of 800 MW. Integrated Gasification Combined Cycle (IGCC) Technology. Launch of Energy Technology Centre -A new initiative for development

of technologies with focus on fundamental R&D.

The company sets aside up to 0.5% of the profits for R&D. Roadmap developed for adopting Clean Development. Mechanism to help get / earn Certified Emission Reduction.

Corporate Social Responsibility

As a responsible corporate citizen NTPC has taken up number of CSRinitiatives.

NTPC Foundation formed to address Social issues at national level NTPC has framed Corporate Social Responsibility Guidelines

committing up to 0.5% of net profit annually for Community Welfare.

The welfare of project affected persons and the local population aroundNTPC projects are taken care of through well drawn Rehabilitation and

Resettlement policies.

The company has also taken up distributed generation for remote ruralareas.

Partnering government in various initiatives

Consultant role to modernize and improvise several plants across thecountry.

Disseminate technologies to other players in the sector. Consultant role Partnership in Excellence Programme for improvement

of PLF of 15 Power Stations of SEBs.

Rural Electrification work under Rajiv Gandhi Garmin Vidyutikaran.

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Environment Management

All stations of NTPC are ISO 14001 certified. Various groups to care of environmental issues. The Environment Management Group. Ash Utilization Division. Afforestation Group. Centre for Power Efficiency & Environment Protection. Group on Clean Development Mechanism. NTPC is the second largest owner of trees in the country after the Forest

department.

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JOURNEY OF NTPC

NTPC was set up in 1975 with 100% ownership by the

Government of India. In the last 30 years, NTPC has

grown into the largest power utility in India.

In 1997, Government of India granted NTPC status of

Navratna being one of the nine jewels of India,

enhancing the powers to the Board of Directors.

NTPC became a listed company with majority

Government ownership of 89.5%.

NTPC becomes third largest by Market Capitalization of

listed companies

The company rechristened as NTPC Limited in line with

its changing business portfolio and transforms itself from

a thermal power utility to an integrated power utility.

National Thermal Power Corporation is the largest power

generation company in India. Forbes Global 2000 for

2008 ranked it 411th in the world.

National Thermal Power Corporation is the largest power

generation company in India. Forbes Global 2000 for

2008 ranked it 317th in the world.

NTPC has also set up a plan to achieve a target of 50,000

MW generation capacity.

NTPC has embarked on plans to become a 75,000 MW

company by 2017.

1975

1997

2005

2004

2008

2009

2017

2012

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ABOUT BTPS

Badarpur thermal power station started working in 1973 with a single 95 mw

unit. There were 2 more units (95 MW each) installed in next 2 consecutiveyears. Now it has total five units with total capacity of 720 MW. Ownership of

BTPS was transferred to NTPC with effect from 01.06.2006 through GOIs

Gazette Notification .Given below are the details of unit with the year they are

installed.

Address: Badarpur, New Delhi110 044

Telephone: (STD-011) - 26949523

Fax: 26949532

Installed Capacity 720 MW

Derated Capacity 705 MW

Location New Delhi

Coal Source Jharia Coal Fields

Water Source Agra Canal

Beneficiary States Delhi

Unit Sizes 3X95 MW

2X210 MW

Units Commissioned Unit I- 95 MW - July 1973

Unit II- 95 MW August 1974

Unit III- 95 MW March 1975

Unit IV - 210 MW December 1978

Unit V - 210 MW - December 1981

Transfer of BTPS to NTPC Ownership of BTPS was transferred to NTPC

with effect from 01.06.2006 through GOIs

Gazette Notification

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BASIC STEPS OF ELECTRICITY GENERATION

The basic steps in the generation of electricity from coal involves following

steps:

Coal to steam Steam to mechanical power Mechanical power to electrical power

COAL TO ELECTRICITY: BASICS

The basic steps in the generation of coal to electricity are shown below:

Coal to Steam

Coal from the coal wagons is unloaded in the coal handling plant. This Coal is

transported up to the raw coal bunkers with the help of belt conveyors. Coal istransported to Bowl mills by Coal Feeders. The coal is pulverized in the Bowl

Mill, where it is ground to powder form. The mill consists of a round metallic

table on which coal particles fall. This table is rotated with the help of a motor.

There are three large steel rollers, which are spaced 120 apart. When there is

no coal, these rollers do not rotate but when the coal is fed to the table it packs

up between roller and the table and ths forces the rollers to rotate. Coal is

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crushed by the crushing action between the rollers and the rotating table. This

crushed coal is taken away to the furnace through coal pipes with the help of hot

and cold air mixture from P.A. Fan.

P.A. Fan takes atmospheric air, a part of which is sent to Air-Preheaters for

heating while a part goes directly to the mill for temperature control.

Atmospheric air from F.D. Fan is heated in the air heaters and sent to the

furnace as combustion air.

Water from the boiler feed pump passes through economizer and reaches the

boiler drum. Water from the drum passes through down comers and goes to the

bottom ring header. Water from the bottom ring header is divided to all the four

sides of the furnace. Due to heat and density difference, the water rises up in the

water wall tubes. Water is partly converted to steam as it rises up in the furnace.

This steam and water mixture is again taken to thee boiler drum where the

steam is separated from water.

Water follows the same path while the steam is sent to superheaters forsuperheating. The superheaters are located inside the furnace and the steam is

superheated (540C) and finally it goes to the turbine.

Flue gases from the furnace are extracted by induced draft fan, which maintains

balance draft in the furnace (-5 to10 mm of wcl) with forced draft fan. These

flue gases emit their heat energy to various super heaters in the pent house and

finally pass through air-preheaters and goes to electrostatic precipitators wherethe ash particles are extracted. Electrostatic Precipitator consists of metal plates,

which are electrically charged. Ash particles are attracted on to these plates, so

that they do not pass through the chimney to pollute the atmosphere. Regular

mechanical hammer blows cause the accumulation of ash to fall to the bottom

of the precipitator where they are collected in a hopper for disposal.

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Steam to Mechanical Power

From the boiler, a steam pipe conveys steam to the turbine through a stop valve

(which can be used to shut-off the steam in case of emergency) and through

control valves that automatically regulate the supply of steam to the turbine.

Stop valve and control valves are located in a steam chest and a governor,

driven from the main turbine shaft, operates the control valves to regulate the

amount of steam used. (This depends upon the speed of the turbine and the

amount of electricity required from the generator).

Steam from the control valves enters the high pressure cylinder of the turbine,

where it passes through a ring of stationary blades fixed to the cylinder wall.

These act as nozzles and direct the steam into a second ring of moving blades

mounted on a disc secured to the turbine shaft. The second ring turns the shafts

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as a result of the force of steam. The stationary and moving blades together

constitute a stage of turbine and in practice many stages are necessary, so that

the cylinder contains a number of rings of stationary blades with rings of

moving blades arranged between them. The steam passes through each stage in

turn until it reaches the end of the high-pressure cylinder and in its passage

some of its heat energy is changed into mechanical energy.

The steam leaving the high pressure cylinder goes back to the boiler for

reheating and returns by a further pipe to the intermediate pressure cylinder.

Here it passes through another series of stationary and moving blades.

Finally, the steam is taken to the low-pressure cylinders, each of which enters at

the centre flowing outwards in opposite directions through the rows of turbine

blades through an arrangement called the double flow- to the extremities of

the cylinder. As the steam gives up its heat energy to drive the turbine, its

temperature and pressure fall and it expands. Because of this expansion the

blades are much larger and longer towards the low pressure ends of the turbine.

Mechanical Power to Electrical Power

As the blades of turbine rotate, the shaft of the generator, which is coupled to

tha of the turbine, also rotates. It results in rotation of the coil of the generator,

which causes induced electricity to be produced.

BASIC POWER PLANT CYCLE

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A simplified diagram of a thermal power plant

The thermal (steam) power plant uses a dual (vapour+ liquid) phase cycle. It is a

close cycle to enable the working fluid (water) to be used again and again. The

cycle used is Rankine Cycle modified to include superheating of steam,

regenerative feed water heating and reheating of steam.

On large turbines, it becomes economical to increase the cycle efficiency by

using reheat, which is a way of partially overcoming temperature limitations.

By returning partially expanded steam, to a reheat, the average temperature at

which the heat is added, is increased and, by expanding this reheated steam to

the remaining stages of the turbine, the exhaust wetness is considerably less

than it would otherwise be conversely, if the maximum tolerable wetness is

allowed, the initial pressure of the steam can be appreciably increased.

Bleed Steam Extraction: For regenerative system, nos. of non-regulated

extractions is taken from HP, IP turbine.

Regenerative heating of the boiler feed water is widely used in modern power

plants; the effect being to increase the average temperature at which heat is

added to the cycle, thus improving the cycle efficiency.

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FACTORS AFFECTING THERMAL CYCLE EFFICIENCY

Thermal cycle efficiency is affected by following:

Initial Steam Pressure. Initial Steam Temperature. Whether reheat is used or not, and if used reheat pressure and

temperature.

Condenser pressure. Regenerative feed water heating.

RANKINE CYCLE

The Rankine cycle is a thermodynamic cycle which converts heat into work.

The heat is supplied externally to a closed loop, which usually uses water as the

working fluid. This cycle generates about 80% of all electric power used

throughout the world, including virtually all solar thermal,biomass, coal and

nuclearpower plants. It is named after William John Macquorn Rankine, a

Scottish polymath..

Description

Physical layout of the four main devices used in the Rankine cycle

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A Rankine cycle describes a model of the operation of steam heat engines most

commonly found in power generation plants. Common heat sources for power

plants using the Rankine cycle are coal, natural gas, oil, and nuclear.

The Rankine cycle is sometimes referred to as a practical Carnot cycle as, when

an efficient turbine is used, the TS diagram will begin to resemble the Carnot

cycle. The main difference is that a pump is used to pressurize liquid instead of

gas. This requires about 1/100th (1%) as much energy as that compressing a gas

in a compressor (as in the Carnot cycle).

The efficiency of a Rankine cycle is usually limited by the working fluid.

Without the pressure going super critical the temperature range the cycle can

operate over is quite small, turbine entry temperatures are typically 565C (the

creep limit of stainless steel) and condenser temperatures are around 30C. This

gives a theoretical Carnot efficiency of around 63% compared with an actual

efficiency of 42% for a modern coal-fired power station. This low turbine entry

temperature (compared with a gas turbine) is why the Rankine cycle is often

used as a bottoming cycle in combined cycle gas turbinepower stations.The working fluid in a Rankine cycle follows a closed loop and is re-used

constantly. The water vapor and entrained droplets often seen billowing from

power stations is generated by the cooling systems (not from the closed loop

Rankine power cycle) and represents the waste heat that could not be converted

to useful work.

Note that cooling towers operate using the latent heat of vaporization of thecooling fluid. The white billowing clouds that form in cooling toweroperation

are the result of water droplets which are entrained in the cooling tower airflow;

it is not, as commonly thought, steam. While many substances could be used in

the Rankine cycle, water is usually the fluid of choice due to its favorable

properties, such as nontoxic and unreactive chemistry, abundance, and low cost,

as well as its thermodynamic properties.

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One of the principal advantages it holds over other cycles is that during the

compression stage relatively little work is required to drive the pump, due to the

working fluid being in its liquid phase at this point. By condensing the fluid to

liquid, the work required by the pump will only consume approximately 1% to

3% of the turbine power and so give a much higher efficiency for a real cycle.

The benefit of this is lost somewhat due to the lower heat addition temperature.

Gas turbines, for instance, have turbine entry temperatures approaching 1500C.

Nonetheless, the efficiencies of steam cycles and gas turbines are fairly well

matched.

Processes of the Rankine cycle

Ts diagram of a typical Rankine cycle operating between pressures of 0.06bar

and 50bar.

There are four processes in the Rankine cycle, each changing the state of the

working fluid. These states are identified by number in the diagram to the right

i. Process 1-2: The working fluid is pumped from low to high pressure, asthe fluid is a liquid at this stage the pump requires little input energy.

ii. Process 2-3: The high pressure liquid enters a boiler where it is heated atconstant pressure by an external heat source to become a dry saturated

vapour.

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iii. Process 3-4: The dry saturated vapor expands through a turbine,generating power. This decreases the temperature and pressure of the

vapor, and some condensation may occur.

iv. Process 4-1: The wet vapor then enters a condenserwhere it is condensedat a constant pressure and temperature to become a saturated liquid. The

pressure and temperature of the condenser is fixed by the temperature of

the cooling coils as the fluid is undergoing a phase-change.

In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the

pump and turbine would generate no entropy and hence maximize the net work

output. Processes 1-2 and 3-4 would be represented by vertical lines on the Ts

diagram and more closely resemble that of the Carnot cycle.

The Rankine cycle shown here prevents the vapor ending up in the superheat

region after the expansion in the turbine, which reduces the energy removed by

the condensers.

Real Rankine cycle (non-ideal) : Rankine cycle with superheat

In a real Rankine cycle, the compression by the pump and the expansion in the

turbine are not isentropic. In other words, these processes are non-reversible and

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entropy is increased during the two processes. This somewhat increases the

powerrequired by the pump and decreases the power generated by the turbine.

In particular the efficiency of the steam turbine will be limited by water droplet

formation. As the water condenses, water droplets hit the turbine blades at high

speed causing pitting and erosion, gradually decreasing the life of turbine blades

and efficiency of the turbine. The easiest way to overcome this problem is by

superheating the steam. On the Ts diagram above, state 3 is above a two phase

region of steam and water so after expansion the steam will be very wet. By

superheating, state 3 will move to the right of the diagram and hence produce a

dryer steam after expansion.

Rankine cycle with reheat

In this variation, two turbines work in series. The first accepts vapor from the

boilerat high pressure. After the vapor has passed through the first turbine, it re-

enters the boiler and is reheated before passing through a second, lower pressure

turbine. Among other advantages, this prevents the vapor from condensingduring its expansion which can seriously damage the turbine blades, and

improves the efficiency of the cycle.

Regenerative Rankine cycle

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The regenerative Rankine cycle is so named because after emerging from the

condenser (possibly as a subcooled liquid) the working fluid is heated by steam

tapped from the hot portion of the cycle. On the diagram shown, the fluid at 2 is

mixed with the fluid at 4 (both at the same pressure) to end up with the saturated

liquid at 7. The Regenerative Rankine cycle (with minor variants) is commonly

used in real power stations.

Another variation is where 'bleed steam' from between turbine stages is sent to

feedwater heaters to preheat the water on its way from the condenser to the

boiler.

I. BOILERMAINTENANCEDEPARTMENTBoiler and Its Description

The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40

m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches

(60 mm) in diameter. Pulverized coal is air-blown into the furnace from fuel

nozzles at the four corners and it rapidly burns, forming a large fireball at the

centre. The thermal radiation of the fireball heats the water that circulates

through the boiler tubes near the boiler perimeter. The water circulation rate in

the boiler is three to four times the throughput and is typically driven by pumps.

As the water in the boiler circulates it absorbs heat and changes into steam at

700 F (370 C) and 3,200 psi (22.1MPa). It is separated from the water inside a

drum at the top of the furnace.

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The saturated steam is introduced into superheat pendant tubes that hang in the

hottest part of the combustion gases as they exit the furnace. Here the steam is

superheated to 1,000 F (540 C) to prepare it for the turbine. The steam

generating boiler has to produce steam at the high purity, pressure and

temperature required for the steam turbine that drives the electrical generator.

The generator includes the economizer, the steam drum, the chemical dosing

equipment, and the furnace with its steam generating tubes and the superheater

coils. Necessary safety valves are located at suitable points to avoid excessive

boiler pressure. The air and flue gas path equipment include: forced draft (FD)

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fan, air preheater (APH), boiler furnace, induced draft (ID) fan, fly ash

collectors (electrostatic precipitator or baghouse) and the flue gas stack.

For units over about 210 MW capacity, redundancy of key components is

provided by installing duplicates of the FD fan, APH, fly ash collectors and ID

fan with isolating dampers. On some units of about 60 MW, two boilers per unit

may instead be provided.

Schematic diagram of a coal-fired power plant steam generator

SPECIFICATIONS OF THE BOILER

1. Main Boiler (AT 100% LOAD):i. Evaporation 700 tons/hr

ii. Feed water temperature 247Ciii. Feed water leaving economizer 276C

2. Steam Temperature:i. Drum 341C

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ii. Super heater outlet 540Ciii. Reheat inlet 332Civ. Reheat outlet 540C

3. Steam Pressure:i. Drum design 158. 20 kg/cm2

ii. Drum operating 149.70 kg/cm2iii. Super heater outlet 137.00 kg/cm2iv. Reheat inlet 26.35 kg/cm2v. Reheat outlet 24.50 kg/cm2

4. Fuel SpecificationsA)Coal

i. Fixed Carbon 38%ii. Volatile Matter 26%

iii. Moisture 8.0%iv. Ash 28%v. Grindability 55HGI

vi. High Heat 4860 Kcal/Kgvii. Coal size to Mill 20 mm

B)Oili. Low Heat value 10000 kcal/kg

ii. Sulphur 4.5% w/wiii. Moisture 1% w/wiv. Flash point 660 C.v. Viscosity 1500 redwood at 37.80 C.

vi. Sp. Weight 0.98 at 380 C.

5. Heat Balance

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i. Dry gas loss 4.63%ii. Carbon loss 2%

iii. Radiation loss 0.26%iv. Unaccounted loss 1.5%v. H

2in air and H

2O in fuel 4.9%

vi. Total loss 13.3%vii. Efficiency 86.7%

AUXILIARIES OF THE BOILER

1. FURNACE Furnace is primary part of boiler where the chemical energy of the fuel is

converted to thermal energy by combustion. Furnace is designed for

efficient and complete combustion. Major factors that assist for efficient

combustion are amount of fuel inside the furnace and turbulence, which

causes rapid mixing between fuel and air. In modern boilers, water

furnaces are used.

TYPES OF FURNACE

P.F. FIRED DRY BOTTOM FURNACE:

The tall rectangular radiant type furnace has now become a feature of modern

dry bottom P.F. boiler. Indorsed height not only facilitates adequate naturalcirculation but also aids reduction of furnace exit gas temperature and henceless soot deposit in superheaters and reheaters.

SLAG TYPE FURNACE:

Furnace of this type normally has two parts. Primary furnace is used for very

high rate of combustion. Provision is to make molten slag and crush the

granular form for easy disposal. As the ash has to flow from the primary

furnace, coal having low melting temperature can only be used. To obtain high

temperature inside the primary surface that will facilitate the easy flow of ash,

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very small but highly rated design is needed for primary furnace hence

maintenance is needed.

OIL FIRED BOILER FURNACE:

Normally about 65% of furnace volume is enough for an oil-fired boiler as

compared to the corresponding P.F. fired boiler.Oil-fired furnace is generally closed at the bottom, as there is no need to remove

slag as in case of P.F. fired boiler. The bottom part will have small amount of

slope to prevent film boiler building in the bottom tubes.

If boiler has to design for both P.F. as well as oil, the furnace has to be designed

for coal, as otherwise higher heat loading with P.F. will cause slogging and highfurnace exit gas temperature.

2. BOILER DRUM Drum is of fusion-welded design with welded hemispherical dished ends.

It is provided with stubs for welding all the connecting tubes, i.e.

downcomers, risers, pipes, saturated steam outlet. The function of steam

drum internals is to separate the water from the steam generated in the

furnace walls and to reduce the dissolved solid contents of the steam

below the prescribed limit of 1 ppm and also take care of the sudden

change of steam demand for boiler.

The secondary stage of two opposite banks of closely spaced thincorrugated sheets, which direct the steam and force the remaining

entertained water against the corrugated plates. Since the velocity isrelatively low this water does not get picked up again but runs down the

plates and off the second stage of the two steam outlets.

From the secondary separators the steam flows upwards to the series ofscreen dryers, extending in layers across the length of the drum. These

screens perform the final stage of the separation.

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Once water inside the boiler or steam generator, the process of adding thelatent heat of vaporization or enthalpy is underway. The boiler transfers

energy to the water by the chemical reaction of burning some type of fuel.

The water enters the boiler through a section in the convection pass calledthe economizer. From the economizer it passes to the steam drum. Once

the water enters the steam drum it goes down the down comers to the

lower inlet water wall headers. From the inlet headers the water rises

through the water walls and is eventually turned into steam due to the

heat being generated by the burners located on the front and rear water

walls (typically). As the water is turned into steam/vapour in the water

walls, the steam/vapour once again enters the steam drum.

External View of an Industrial Boiler at BTPS, New Delhi

The steam/vapour is passed through a series of steam and waterseparators and then dryers inside the steam drum. The steam separators

and dryers remove the water droplets from the steam and the cycle

through the water walls is repeated. This process is known as natural

circulation.

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The boiler furnace auxiliary equipment includes coal feed nozzles andigniter guns, soot blowers, water lancing and observation ports (in the

furnace walls) for observation of the furnace interior. Furnace explosions

due to any accumulation of combustible gases after a tripout are avoided

by flushing out such gases from the combustion zone before igniting the

coal.

The steam drum (as well as the superheater coils and headers) have airvents and drains needed for initial start-up. The steam drum has an

internal device that removes moisture from the wet steam entering the

drum from the steam generating tubes. The dry steam then flows into the

superheater coils. Geothermal plants need no boiler since they use

naturally occurring steam sources.

Heat exchangers may be used where the geothermal steam is verycorrosive or contains excessive suspended solids. Nuclear plants also boil

water to raise steam, either directly passing the working steam through

the reactor or else using an intermediate heat exchanger.

3. WATER WALLS Water flows to the water walls from the boiler drum by natural

circulation. The front and the two side water walls constitute the main

evaporation surface, absorbing the bulk of radiant heat of the fuel burnt inthe chamber. The front and rear walls are bent at the lower ends to form a

water-cooled slag hopper. The upper part of the chamber is narrowed to

achieve perfect mixing of combustion gases. The water wall tubes are

connected to headers at the top and bottom. The rear water wall tubes at

the top are grounded in four rows at a wider pitch forming g the grid

tubes.

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4. REHEATER Reheater is used to raise the temperature of steam from which a part of

energy has been extracted in highpressure turbine. This is another

method of increasing the cycle efficiency. Reheating requires additional

equipment i.e. heating surface connecting boiler and turbine pipe safety

equipment like safety valve, non return valves, isolating valves, high

pressure feed pump, etc: Reheater is composed of two sections namely

the front and the rear pendant section, which is located above the furnace

arc between water-cooled, screen wall tubes and rear wall tubes.

Tubes of a reheater

5. SUPERHEATER Whatever type of boiler is used, steam will leave the water at its surface

and pass into the steam space. Steam formed above the water surface in a

shell boiler is always saturated and become superheated in the boiler

shell, as it is constantly. If superheated steam is required, the saturated

steam must pass through a superheater. This is simply a heat exchanger

where additional heat is added to the steam.

In water-tube boilers, the superheater may be an additional pendantsuspended in the furnace area where the hot gases will provide the degree

of superheat required. In other cases, for example in CHP schemes where

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the gas turbine exhaust gases are relatively cool, a separately fired

superheater may be needed to provide the additional heat.

6. ECONOMIZER The function of an economizer in a steam-generating unit is to absorb

heat from the flue gases and add as a sensible heat to the feed water

before the water enters the evaporation circuit of the boiler.

Earlier economizer were introduced mainly to recover the heat availablein the flue gases that leaves the boiler and provision of this addition

heating surface increases the efficiency of steam generators. In the

modern boilers used for power generation feed water heaters were used to

increase the efficiency of turbine unit and feed water temperature.

An economizer

Use of economizer or air heater or both is decided by the total economythat will result in flexibility in operation, maintenance and selection of

firing system and other related equipment. Modern medium and high

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capacity boilers are used both as economizers and air heaters. In low

capacity, air heaters may alone be selected.

Stop valves and non-return valves may be incorporated to keepcirculation in economizer into steam drum when there is fire in the

furnace but not feed flow. Tube elements composing the unit are built up

into banks and these are connected to inlet and outlet headers.

7. AIR PREHEATER Air preheater absorbs waste heat from the flue gases and transfers this

heat to incoming cold air, by means of continuously rotating heat transfer

element of specially formed metal plates. Thousands of these high

efficiency elements are spaced and compactly arranged within 12

sections. Sloped compartments of a radially divided cylindrical shell

called the rotor. The housing surrounding the rotor is provided with duct

connecting both the ends and is adequately scaled by radial and

circumferential scaling.

An air preheater

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Special sealing arrangements are provided in the provided in the airpreheater to prevent the leakage between the air and gas sides. Adjustable

plates are also used to help the sealing arrangements and prevent the

leakage as expansion occurs. The air preheater heating surface elements

are provided with two types of cleaning devices, soot blowers to clean

normal devices and washing devices to clean the element when soot

blowing alone cannot keep the element clean.

8. PULVERIZER A pulverizer is a mechanical device for the grinding of many types of

materials. For example, they are used to pulverize coal for combustion in

the steam-generating furnaces of the fossil fuel power plants.

A Pulverizer

Types of Pulverizer

i. Ball and Tube mills

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A ball mill is a pulverizer that consists of a horizontal cylinder, up to

three diameters in length, containing a charge of tumbling or cascading

steel balls, pebbles or steel rods.

A tube mill is a revolving cylinder of up to five diameters in length used

for finer pulverization of ore, rock and other such materials; the materials

mixed with water is fed into the chamber from one end, and passes out

the other end as slime.

ii. Bowl millIt uses tires to crush coal. It is of two types; a deep bowl mill and the

shallow bowl mill.

An external view of a Coal Pulverizer

Advantages of Pulverized Coal

Pulverized coal is used for large capacity plants. It is easier to adapt to fluctuating load as there are no limitations on the

combustion capacity.

Coal with higher ash percentage cannot be used without pulverizingbecause of the problem of large amount ash deposition after combustion.

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Increased thermal efficiency is obtained through pulverization. The use of secondary air in the combustion chamber along with the

powered coal helps in creating turbulence and therefore uniform mixing

of the coal and the air during combustion.

Greater surface area of coal per unit mass of coal allows fastercombustion as more coal is exposed to heat and combustion.

The combustion process is almost free from clinker and slag formation. The boiler can be easily started from cold condition in case of emergency. Practically no ash handling problem. The furnace volume required is less as the turbulence caused aids in

complete combustion of the coal with minimum travel of the particles.

Basics of Fans

The air we need for combustion in the furnace and the flue gas that we must

evacuate would not possible without using fans. A fan is capable of impartingenergy to the air/gas in the form of a boost in pressure. We overcome the

losses through the system by means of this pressure boost. The boost is

dependent on density for a given fan at a given speed. The higher the

temperature, the lower is the boost. Fan performance (Max. capability) is

represented as volume vs. pressure boost.

The basic information needed to select a fan is:

Air or Gas flow (Kg/hr). Density (function of temperature and pressure). System, resistance (losses).

Classification of Fans

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In boiler practice, we meet the following types of fans.

Axial fans Centrifugal (Radial) fans

Axial Fans

In this type the movement of air or gas is parallel to its exit of rotation. These

fans are better suited to low resistance applications. The axial flow fan uses the

screw like action of a multiplied rotating shaft, or propeller, to move air or gas

in a straight through path.

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Centrifugal Fan

This fan moves gas or air perpendicular to the axis of

rotation. There are advantages when the air must be moved in a system where

the frictional resistance is relatively high. The blade wheel whirls air

centrifugally between each pair of blades and forces it out peripherally at high

velocity and high static pressure. More air is sucked in at the eye of the

impeller. As the air leaves the revolving blade tips, part of its velocity is

converted into additional static pressure by scroll shaped housing.

There are three types of blades.

Backward curved blades. Forward curved blades. Radial blades.

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Draft System

Before a detailed study of industrial fans it is in the fitness of things to

understand the various draft systems maintained by those fans.

The terms draft denotes the difference between the

atmospheric pressure and the pressure existing in the furnace.

Depending upon the draft used, we have

Natural Draft Induced Draft Forced Draft Balanced Draft System

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Natural Draft

In natural draft units the pressure differentials are

obtained have constructing tall chimneys so that vacuum is' created in thefurnace Due to small pressure difference, air is admitted into the furnace.

Induced Draft

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In this system the air is admitted to natural pressure

difference and the flue gases are taken out by means of induced Draft fans and

the furnace is maintained under vacuum.

Forced Draft

A set of forced draft fans are made use of for supplying air

to the furnace and so the furnace is pressurized. The flue gases are taken out

due to the pressure difference between the furnace and the atmosphere.

Balance Draft

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Here a set of Induced and Forced Draft Fans are utilized in maintaining a

vacuuming the furnace. Normally all the power stations utilize this draft

system.

Industrial Fans

I.D. Fan

The induced Draft Fans are generally of Axial -Impulse Type. Impeller nominal

diameter is of the order of 2500 mm.

The fan consists of the following sub-assemblies

Suction Chamber Inlet Vane Control Impeller Outlet Guide Vane Assembly

The outlet guides are fixed in between the case of the diffuser and the casing.

These guide vanes serve to direct the flow axially and to stabilize the draft-flow

caused in the impeller. These outlet blades are removable type from outside.

During operation of the fan itself these blades can be replaced one by one.

Periodically the outlet blades can be removed one at a time to find out the

extent of wear on the blade. If excessive wear is noticed the blade can be

replaced by a new blade.

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F.D Fan

The fan, normally of the same type as ID Fan, consists of the following

components:

* Silencer

* Inlet bend

* Fan housing

* Impeller with blades and setting mechanism

* Guide wheel casing with guide vanes and diffuser.

The centrifugal and setting forces of the blades are taken up by the blade

bearings. The blade shafts are placed in combined radial and axial antifriction

bearings which are sealed off to the outside. The angle of-incidence of the

blades may be adjusted during operation. The characteristic pressure volume

curves of the fan may be changed in a large range without essentially

modifying the efficiency. The fan can then be easily adapted to changing

operating conditions.

The rotor is accommodated in cylindrical roller bearings and an inclined ball

bearing at the drive side adsorbs the axial thrust.

Lubrication and cooling these bearings is assured by a combined oil level and

circulating lubrication system.

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iii. Combined circulation system

i. Natural Circulation SystemWater delivered to steam generator from feed water is at a temperature well

below the saturation value corresponding to that pressure. Entering first the

economizer, it is heated to about 30-40C below saturation temperature. From

economizer the water enters the drum and thus joins the circulation system.

Water entering the drum flows through the down corner and enters ring heater

at the bottom. In the water walls, a part of the water is converted to steam and

the mixture flows back to the drum. In the drum, the steam is separated, and

sent to superheater for superheating and then sent to the high-pressure turbine.

Remaining water mixes with the incoming water from the economizer and the

cycle is repeated.

As the pressure increases, the difference in density between water and steam

reduces. Thus the hydrostatic head available will not be able to overcome thefrictional resistance for a flow corresponding to the minimum requirement of

cooling of water wall tubes. Therefore natural circulation is limited to the boiler

with drum operating pressure around 175 kg/ cm2.

ii. Controlled Circulation SystemBeyond 80 kg/ cm

2of pressure, circulation is to be assisted with mechanical

pumps to overcome the frictional losses. To regulate the flow through various

tubes, orifices plates are used. This system is applicable in the high sub-critical

regions (200 kg/ cm2).

2. ASH HANDLING PLANTThe widely used ash handling systems are:

i. Mechanical Handling System

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ii. Hydraulic Systemiii. Pneumatic Systemiv. Steam Jet System

Ash Handling System at Badarpur Thermal Power Station, New Delhi

The Hydraulic Ash handling system is used at the Badarpur Thermal Power

Station.

Hydraulic Ash Handling System

The hydraulic system carried the ash with the flow of water with high velocity

through a channel and finally dumps into a sump. The hydraulic system is

divided into a low velocity and high velocity system. In the low velocity system

the ash from the boilers falls into a stream of water flowing into the sump. The

ash is carried along with the water and they are separated at the sump. In the

high velocity system a jet of water is sprayed to quench the hot ash. Two other

jets force the ash into a trough in which they are washed away by the water into

the sump, where they are separated. The molten slag formed in the pulverized

fuel system can also be quenched and washed by using the high velocity system.

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The advantages of this system are that its clean, large ash handling capacity,

considerable distance can be traversed, absence of working parts in contact with

ash.

Fly Ash Collection

Fly ash is captured and removed from the flue gas by electrostatic precipitators

or fabric bag filters (or sometimes both) located at the outlet of the furnace and

before the induced draft fan. The fly ash is periodically removed from the

collection hoppers below the precipitators or bag filters. Generally, the fly ash is

pneumatically transported to storage silos for subsequent transport by trucks orrailroad cars.

Bottom Ash Collection and Disposal

At the bottom of every boiler, a hopper has been provided for collection of the

bottom ash from the bottom of the furnace. This hopper is always filled with

water to quench the ash and clinkers falling down from the furnace. Some

arrangement is included to crush the clinkers and for conveying the crushed

clinkers and bottom ash to a storage site.

3. WATER TREATMENT PLANTAs the types of boiler are not alike their working pressure and operating

conditions vary and so do the types and methods of water treatment. Water

treatment plants used in thermal power plants used in thermal power plants are

designed to process the raw water to water with a very low content of dissolved

solids known as demineralized water. No doubt, this plant has to be

engineered very carefully keeping in view the type of raw water to the thermal

plant, its treatment costs and overall economics.

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A water treatment plant

The type of demineralization process chosen for a power station depends on

three main factors:

i. The quality of the raw water.ii. The degree of de-ionization i.e. treated water quality.iii. Selectivity of resins.

Water treatment process is generally made up of two sections:

Pretreatment section. Demineralization section

Pretreatment Section

Pretreatment plant removes the suspended solids such as clay, silt, organic and

inorganic matter, plants and other microscopic organism. The turbidity may be

taken as two types of suspended solid in water; firstly, the separable solids and

secondly the non-separable solids (colloids). The coarse components, such as

sand, silt, etc: can be removed from the water by simple sedimentation. Finer

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particles, however, will not settle in any reasonable time and must be

flocculated to produce the large particles, which are settle able. Long term

ability to remain suspended in water is basically a function of both size and

specific gravity.

Demineralization

This filter water is now used for demineralizing purpose and is fed to cation

exchanger bed, but enroute being first dechlorinated, which is either done by

passing through activated carbon filter or injecting along the flow of water, an

equivalent amount of sodium sulphite through some stroke pumps. The residualchlorine, which is maintained in clarification plant to remove organic matter

from raw water, is now detrimental to action resin and must be eliminated

before its entry to this bed.

A demineralization tank

A DM plant generally consists of cation, anion and mixed bed exchangers. The

final water from this process consists essentially of hydrogen ions and

hydroxide ions which is the chemical composition of pure water. The DM

water, being very pure, becomes highly corrosive once it absorbs oxygen from

the atmosphere because of its very high affinity for oxygen absorption. The

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capacity of the DM plant is dictated by the type and quantity of salts in the raw

water input. However, some storage is essential as the DM plant may be down

for maintenance. For this purpose, a storage tank is installed from which DM

water is continuously withdrawn for boiler make-up. The storage tank for DM

water is made from materials not affected by corrosive water, such as PVC. The

piping and valves are generally of stainless steel. Sometimes, a steam

blanketing arrangement or stainless steel doughnut float is provided on top of

the water in the tank to avoid contact with atmospheric air. DM water make-up

is generally added at the steam space of the surface condenser (i.e., the vacuum

side). This arrangement not only sprays the water but also DM water gets

deaerated, with the dissolved gases being removed by the ejector of the

condenser itself.

4. DRAUGHT SYSTEMThere are four types of draught system:

i. Natural Draughtii. Induced Draught

iii. Forced Draughtiv. Balanced Draught

Natural Draught System

In natural draft units the pressure differentials are obtained have constructing

tail chimneys so that vacuum is created in the furnace. Due to small pressure

difference, air is admitted into the furnace.

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A natural draught system

Induced Draft System

In this system, the air is admitted to natural pressure difference and the flue

gases are taken out by means of Induced Draught (I.D.) fans and the furnace is

maintained under vacuum.

An induced draught system

Forced Draught System

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A set of forced draught (F.D.) fans is made use of for supplying air to the

furnace and so the furnace is pressurized. The flue gases are taken out due to the

pressure difference between the furnace and the atmosphere.

A forced draught system

Balanced Draught System

Here a set of Induced and Forced Draft Fans are utilized in maintaining a

vacuum in the furnace. Normally all the power stations utilize this draft system.

5. INDUSTRIAL FANSID Fan

The induced Draft Fans are generally of Axial-Impulse Type. Impeller nominal

diameter is of the order of 2500 mm. The fan consists of the following sub-

assemblies:

Suction Chamber Inlet Vane Control Impeller

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curves of the fan may be changed in a large range without essentially modifying

the efficiency. The fan can then be easily adapted to changing operating

conditions.

The rotor is accommodated in cylindrical roller bearings and an inclined ball

bearing at the drive side absorbs the axial thrust.

Lubrication and cooling these bearings is assured by a combined oil level and

circulating lubrication system.

Primary Air Fan

PA Fan if flange-mounted design, single stage suction, NDFV type, backward

curved bladed radial fan operating on the principle of energy transformation due

to centrifugal forces. Some amount of the velocity energy is converted to

pressure energy in the spiral casing. The fan is driven at a constant speed and

varying the angle of the inlet vane control controls the flow. The special feature

of the fan is that is provided with inlet guide vane control with a positive and

precise link mechanism.

It is robust in construction for higher peripheral speed so as to have unit sizes.

Fan can develop high pressures at low and medium volumes and can handle

hot-air laden with dust particles.

Primary air fan

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6. COMPRESSOR HOUSEInstrument air is required for operating various dampers, burner tilting, devices,

diaphragm valves, etc: in the 210 MW units. Station air meets the general

requirement of the power station such as light oil atomizing air, for cleaning

filters and for various maintenance works. The control air compressors and

station air compressors have been housed separately with separate receivers and

supply headers and their tapping.

A compressor house

Instrument Air System

Control air compressors have been installed for supplying moisture free dry air

required for instrument used. The output from the compressors is fed to air

receivers via return valves. From the receiver air passed through the dryers to

the main instrument airline, which runs along with the boiler house and turbine

house of 210 MW units. Adequate numbers of tapping have been provided all

over the area.

Air-Drying Unit

Air contains moisture which tends to condense, and causes trouble in operation

of various devices by compressed air. Therefore drying of air is accepted widely

in case of instrument air. Air drying unit consists of dual absorption towers with

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embedded heaters for reactivation. The absorption towers are adequately filled

with specially selected silica gel and activated alumina while one tower is

drying the air.

An air drying unit

Service Air Compressor

The station air compressor is generally a slow speed horizontal double acting

double stage type and is arranged for belt drive. The cylinder heads and barrel

are enclosed in a jacket, whih extends around the valve also. The intercooler is

provided between the low and high pressure cylinder which cools the air

between tag and collects the moisture that condenses.

A service air compressor

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Air from L.P. cylinder enters at one end of the intercooler and goes to the

opposite end wherefrom it is discharged to the high-pressure cylinder; cooling

water flows through the nest of the tubes and cools the air. A safety valve is set

at rated pressure.

Two selector switches one with positions auto load/unload and another with

positions auto start/stop, non-stop have been provided on the control panel of

the compressor. In auto start-stop position, the compressor will start.

III. TURBINE MAINTENANCE DEPARTMENTTURBINE CLASSIFICATION:

1. Impulse turbine:In impulse turbine steam expands in fixed nozzles. The high velocity

steam from nozzles does work on moving blades, which causes the shaft

to rotate. The essential features of impulse turbine are that all pressure

drops occur at nozzles and not on blades.

2. Reaction turbine:In this type of turbine pressure is reduced at both fixed and moving

blades. Both fixed and moving blades act like nozzles. Work done by the

impulse effect of steam due to reverse the direction of high velocity

steam. The expansion of steam takes place on moving blades.

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A 95 MW Generator at BTPS, New Delhi

COMPOUNDING:

Several problems occur if energy of steam is converted in single step and so

compounding is done. Following are the type of compounded turbine:

i. Velocity Compounded Turbine:Like simple turbine it has only one set of nozzles and entire steam

pressure drop takes place there. The kinetic energy of steam fully on

the nozzles is utilized in moving blades. The role of fixed blades is to

change the direction of steam jet and too guide it.

ii. Pressure Compounded Turbine:This is basically a number of single impulse turbines in series or on

the same shaft. The exhaust of first turbine enters the nozzles of next

turbine. The total pressure drop of steam does not tae on first nozzle

ring but divided equally on all of them.

iii. Pressure Velocity Compounded Turbine:

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It is just the combination of the two compounding and has the

advantages of allowing bigger pressure drops in each stage and so

fewer stages are necessary. Here for given pressure drop the turbine

will be shorter length but diameter will be increased.

MAIN TURBINE:

The 210MW turbine is a cylinder tandem compounded type machine

comprising of H.P. and I.P and L.P cylinders. The H.P. turbine comprises of 12

stages the I.P turbine has 11 stages and the L.P has four stages of double flow.

The H.P and I.P. turbine rotor are rigidly compounded and the I.P. and L.P rotor

by lens type semi flexible coupling. All the 3 rotors are aligned on five bearings

of which the bearing number is combined with thrust bearing.

The main superheated steam branches off into two streams from the boiler and

passes through the emergency stop valve and control valve before entering the

governing wheel chamber of the H.P. Turbine. After expanding in the 12 stages

in the H.P. turbine then steam is returned in the boiler for reheating.The reheated steam from boiler enters I.P. turbine via the interceptor valves and

control valves and after expanding enters the L.P stage via 2 numbers of cross

over pipes.

In the L.P. stage the steam expands in axially opposed direction to counteract

the thrust and enters the condenser placed directly below the L.P. turbine. The

cooling water flowing through the condenser tubes condenses the steam and thecondensate the collected in the hot well of the condenser.

The condensate collected the pumped by means of 3x50% duty condensate

pumps through L.P heaters to deaerator from where the boiler feed pump

delivers the water to the boiler through H.P. heaters thus forming a closed cycle.

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STEAM TURBINE

A steam turbine is a mechanical device that extracts thermal energy from

pressurized steam and converts it into useful mechanical work.

From a mechanical point of view, the turbine is ideal, because the propelling

force is applied directly to the rotating element of the machine and has not as in

the reciprocating engine to be transmitted through a system of connecting links,

which are necessary to transform a reciprocating motion into rotary motion.

Hence since the steam turbine possesses for its moving parts rotating elements

only if the manufacture is good and the machine is correctly designed, it ought

to be free from out of balance forces.

If the load on a turbine is kept constant the torque developed at the coupling is

also constant. A generator at a steady load offers a constant torque. Therefore, a

turbine is suitable for driving a generator, particularly as they are both high-

speed machines.

A further advantage of the turbine is the absence of internal lubrication. This

means that the exhaust steam is not contaminated with oil vapour and can becondensed and fed back to the boilers without passing through the filters. It also

means that turbine is considerable saving in lubricating oil when compared with

a reciprocating steam engine of equal power.

A final advantage of the steam turbine and a very important one is the fact that a

turbine can develop many time the power compared to a reciprocating engine

whether steam or oil.

OPERATING PRINCIPLES

A steam turbines two main parts are the cylinder and the rotor. The cylinder

(stator) is a steel or cast iron housing usually divided at the horizontal

centerline. Its halves are bolted together for easy access. The cylinder contains

fixed blades, vanes and nozzles that direct steam into the moving blades carried

by the rotor. Each fixed blade set is mounted in diaphragms located in front of

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each disc on the rotor, or directly in the casing. A disc and diaphragm pair a

turbine stage. Steam turbines can have many stages. A rotor is a rotating shaft

that carries the moving blades on the outer edges of either discs or drums. The

blades rotate as the rotor revolves. The rotor of a large steam turbine consists of

large, intermediate and low-pressure sections.

In a multiple-stage turbine, steam at a high pressure and high temperature enters

the first row of fixed blades or nozzles through an inlet valve/valves. As the

steam passes through the fixed blades or nozzles, it expands and its velocity

increases. The high velocity jet of stream strikes the first set of moving blades.

The kinetic energy of the steam changes into mechanical energy, causing the

shaft to rotate. The steam that enters the next set of fixed blades strikes the next

row of moving blades.

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As the steam flows through the turbine, its pressure and temperature decreases

while its volume increases. The decrease in pressure and temperature occurs as

the steam transmits energy to the shaft and performs work. After passing

through the last turbine stage, the steam exhausts into the condenser or process

steam system.

The kinetic energy of the steam changes into mechanical energy through the

impact (impulse) or reaction of the steam against the blades. An impulse turbine

uses the impact force of the steam jet on the blades to turn the shaft. Steam

expands as it passes through thee nozzles, where its pressure drops and its

velocity increases. As the steam flows through the moving blades, its pressure

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remains the same, but its velocity decreases. The steam does not expand as it

flows through the moving blades.

STEAM CYCLE

The thermal (steam) power plant uses a dual (vapor+liquid) phase cycle. It is a

closed cycle to enable the working fluid (water) to be used again and again. The

cycle used is Rankine cycle modified to include superheating of steam,

regenerative feed water heating and reheating of steam.

MAIN TURBINE

The 210 MW turbine is a tandem compounded type machine comprising of H.P.

and I.P. cylinders. The H.P. turbines comprise of 12 stages, I.P. turbine has 11stages and the L.P. turbine has 4 stages of double flow.

The H.P. and I.P. turbine rotors are rigidly compounded and the L.P. motor by

the lens type semi flexible coupling. All the three rotors are aligned on five

bearings of which the bearing no. 2 is combined with the thrust bearing

The main superheated steam branches off into two streams from the boiler and

passes through the emergency stop valve and control valve before entering the

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governing wheel chamber of the H.P. turbine. After expanding in the 12 stages

in the H.P. turbine the steam is returned in boiler for reheating.

The reheated steam for the boiler enters the I.P> turbine via the interceptor

valves and control valves and after expanding enters the L.P. turbine stage via 2

nos of cross-over pipes.

In the L.P. stage the steam expands in axially opposite direction to counteract

the trust and enters the condensers placed below the L.P. turbine. The cooling

water flowing throughout the condenser tubes condenses the steam and the

condensate collected in the hot well of the condenser.

The condensate collected is pumped by means of 3*50% duty condensate

pumps through L.P. heaters to deaerator from where the boiler feed pump

delivers the water to boiler through H.P. heaters thus forming a close cycle.

The Main Turbine

TURBINE CYCLE

Fresh steam from the boiler is supplied to the turbine through the emergency

stop valve. From the stop valves steam is supplied to control valves situated in

H.P. cylinders on the front bearing end. After expansion through 12 stages at the

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H.P. cylinder, steam flows back to the boiler for reheating steam and reheated

steam from the boiler cover to the intermediate pressure turbine through two

interceptor valves and four control valves mounted on I.P. turbine.

After flowing through I.P. turbine steam enters the middle part of the L.P.

turbine through cross-over pipes. In L.P. turbine the exhaust steam condenses in

the surface condensers welded directly to the exhaust part of L.P. turbine.

The Turbine Cycle

The selection of extraction points and cold reheat pressure has been done with a

view to achieve a high efficiency. These are two extractors from H.P. turbine,

four from I.P. turbine and one from L.P. turbine. Steam at 1.10 and 1.03 g/sq.

cm. Abs is supplied for the gland sealing. Steam for this purpose is obtained

from deaerator through a collection where pressure of steam is regulated.

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From the condenser, condensate is pumped with the help of 3*50% capacity

condensate pumps to deaerator through the low-pressure regenerative

equipments.

Feed water is pumped from deaerator to the boiler through the H.P. heaters by

means of 3*50% capacity feed pumps connected before the H.P. heaters.

SPECIFICATIONS OF THE TURBINE

Type: Tandem compound 3 cylinder reheated type. Rated power: 210 MW. Number of stages: 12 in H.P., 11 in I.P. and 4*2 in L.P. cylinder. Rated steam pressure: 130 kg /sq. cm before entering the stop valve. Rated steam temperature: 535C after reheating at inlet. Steam flow: 670T / hr. H.P. turbine exhaust pressure: 27 kg /sq. cm., 327C Condenser back pressure: 0.09 kg /sq. cm. Type of governing: nozzle governing. Number of bearing; 5 excluding generator and exciter. Lubrication Oil: turbine oil 14 of IOC. Gland steam pressure: 1.03 to 1.05 kg /sq. cm (Abs) Critical speed: 1585, 1881, 2017. Ejector steam parameter: 4.5 kg /sq. cm. Condenser cooling water pressure: 1.0 to 1.1 kg /sq. cm. Condenser cooling water temperature: 27000 cu. M /hr. Number of extraction lines for regenerative heating of feed water;

seven.

TURBINE COMPONENTS

Casing.

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Rotor. Blades. Sealing system. Stop & control valves. Couplings and bearings. Barring gear.

TURBINE CASINGS

HP Turbine Casings:

Outer casing: a barrel-type without axial or radial flange. Barrel-type casing suitable for quick startup and loading. The inner casing- cylindrically, axially split. The inner casing is attached in the horizontal and vertical planes in the

barrel casing so that it can freely expand radially in all the directions and

axially from a fixed point (HP- inlet side).

IP Turbine Casing:

The casing of the IP turbine is split horizontally and is of double-shellconstruction.

Both are axially split and a double flow inner casing is supported in theouter casing and carries the guide blades.

Provides opposed double flow in the two blade sections and compensatesaxial thrust.

Steam after reheating enters the inner casing from Top & Bottom.

LP Turbine Casing:

The LP turbine casing consists of a double flow unit and has a triple shellwelded casing.

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The shells are axially split and of rigid welded construction. The inner shell taking the first rows of guide blades is attached

kinematically in the middle shell.

Independent of the outer shell, the middle shell, is supported at fourpoints on longitudinal beams.

Steam admitted to the LP turbine from the IP turbine flows into the innercasing from both sides.

ROTORS

HP Rotor:

The HP rotor is machined from a single Cr-Mo-V steel forging withintegral discs.

In all the moving wheels, balancing holes are machined to reduce thepressure difference across them, which results in reduction of axial thrust.

First stage has integral shrouds while other rows have shroudings, rivetedto the blades are periphery.

IP Rotor:

The IP rotor has seven discs integrally forged with rotor while last fourdiscs are shrunk fit.

The shaft is made of high creep resisting Cr-Mo-V steel forging while theshrunk fit discs are machined from high strength nickel steel forgings.

Except the last two wheels, all other wheels have shrouding riveted at thetip of the blades. To adjust the frequency of thee moving blades, lashing

wires have been provided in some stages.

LP Rotor:

The LP rotor consists of shrunk fit discs in a shaft.

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The shaft is a forging of Cr-Mo-V steel while the discs are of highstrength nickel steel forgings.

Blades are secured to the respective discs by riveted fork root fastening. In all the stages lashing wires are provided to adjust the frequency of

blades. In the last two rows, satellite strips are provided at the leading

edges of the blades to protect them against wet-steam erosion.

BLADES

Most costly element of the turbine. Blades fixed in stationary part are called guide blades/ nozzles and those

fitted in moving part are called rotating/working blades.

Blades have three main parts:o Aerofoil: working part.o Root.o Shrouds.

Shroud are used to prevent steam leakage and guide steam to next set ofmoving blades.

VACUUM SYSTEM

This comprises of:

Condenser: 2 for 200 MW unit at the exhaust of LP turbine. Ejectors: One starting and two main ejectors connected to the condenser

locared near the turbine.

C.W. Pumps:Normally two per unit of 50% capacity.

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CONDENSER

There are two condensers entered to the two exhausters of the L.P. turbine.

These are surface-type condensers with two pass arrangement. Cooling water

pumped into each condenser by a vertical C.W. pump through the inlet pipe.

Water enters the inlet chamber of the front water box, passes horizontally

through brass tubes to the water tubes to the water box at the other end, takes a

turn, passes through the upper cluster of tubes and reaches the outlet chamber in

the front water box. From these, cooling water leaves the condenser through the

outlet pipe and discharge into the discharge duct.

Steam exhausted from the LP turbine washes the outside of the condenser tubes,

losing its latent heat to the cooling water and is connected with water in the

steam side of the condenser. This condensate collects in the hot well, welded to

the bottom of the condensers.

A typical water cooled condensor

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EJECTORS

There are two 100% capacity ejectors of the steam eject type. The purpose of

the ejector is to evacuate air and other non-condensating gases from the

condensers and thus maintain the vacuum in the condensers.

The ejector has three compartments. Steam is supplied generally at a pressure of

4.5 to 5 kg /cm2 to the three nozzles in the three compartments. Steam expands

in the nozzle thus giving a high-velocity eject which creates a low-pressure zone

in the throat of the eject. Since the nozzle box of the ejector is connected to the

air pipe from the condenser, the air and pressure zone. The working steam

which has expanded in volume comes into contact with the cluster of tube

bundles through which condensate is flowing and gets condensed thus after

aiding the formation of vacuum. The non-condensing gases of air are further

sucked with the next stage of the ejector by the second nozzle. The process

repeats itself in the third stage also and finally the steam-air mixture is

exhausted into the atmosphere through the outlet.

CONDENSATE SYSTEM

This contains the following

i. Condensate Pumps: 3 per unit of 50% capacity each located nearcondenser hot well.

ii. LP Heater: Normally 4 in number with no.1 located at the upper part ofthe condenser and nos. 2,3 & 4 around 4m level.

iii. Deaerator;one per unit located around 181 M level in CD bay.

Condensate Pumps

The function of these pumps is to pump out the condensate to the desecrator

through ejectors, gland steam cooler and LP heaters. These pumps have four

stages and since the suction is at a negative pressure, special arrangements have

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been made for providing sealing. The pump is generally rated for 160 m3/ hr at a

pressure of 13.2 kg/ cm2 .

L.P. Heaters

Turbine has been provided with non-controlled extractions, which are utilized

for heating the condensate, from turbine bleed steam. There are 410 W pressure

heaters in which the last four extractions are used. L.P. Heater-1 has two parts

LPH-1A and LPH-1B located in the upper parts of the condenser A and

condenser B, respectively. These are of horizontal type with shell and tube

construction. L.P.H. 2,3 and 4 are of similar construction and they are mounted

in a row of 5m level. They are of vertical construction with brass tubes the ends

of which are expanded into tube plate. The condensate flows in the U tubes in

four passes and extraction steam washes the outside of the tubes. Condensate

passes through these four L.P. heaters in succession. These heaters are equipped

with necessary safety valves in the steam space level indicator for visual level

indication of heating steam condensate pressure vacuum gauges formeasurement of steam pressure, etc:

Deaerator

The presence of certain gases, principally oxygen, carbon dioxide and ammonia,

dissolved in water is generally considered harmful because of their corrosive

attack on metals, particularly at elevated temperatures. One of the mostimportant factors in the prevention of internal corrosion in modern boilers and

associated plant therefore, is that the boiler feed water should be free as far as

possible from all dissolved gases especially oxygen. This is achieved by

embodying into the boiler feed system a deaerating unit, whose function is to

remove the dissolved gases from the feed water by mechanical means.

Particularly the unit must reduce the oxygen content of the feed water to a lower

value as far as possible, depending upon the individual circumstances. Residual

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oxygen content in condensate at the outlet of deaerating plant usually specified

are 0.005/ litre or less.

A Deaerator

PRINCIPAL OF DEAERATION

It is based on following two laws.

Henrys Law Solubility

The Deaerator comprises of two chambers:

Deaerating column Feed storage tank

Deaerating column is a spray cum tray type cylindrical vessel of horizontal

construction with dished ends welded to it. The tray stack is designed to ensure

maximum contact time as well as optimum scrubbing of condensate to achieve

efficient deaeration. The deaeration column is mounted on the feed storage tank,

which in turn is supported on rollers at the two ends and a fixed support at the

centre. The feed storage tank is fabricated from boiler quality steel plates.

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Manholes are provided on deaerating column as well as on feed storage tank for

inspection and maintenance.

The condensate is admitted at the top of the deaerating column flows

downwards through the spray valves and trays. The trays are designed to expose

to the maximum water surfaces for efficient scrubbing to affect the liberation of

the associated gases steam enters from the underneath of the trays and flows in

counter direction of condensate. While flowing upwards through the trays,

scrubbing and heating is done. Thus the liberated gases move upwards

alongwith the steam. Steam gets condensed above the trays and in turn heats the

condensate. Liberated gases escapes to atmosphere from the orifice opening

meant for it. This opening is provided with a number of dlflectors to minimize

the loss of steam.

FEED WATER SYSTEM

The main equipments coming under this system are:

Boiler feed Pump: Three perunit of 50% capacity each located in the 0meter level in the T bay.

High Pressure Heaters: Normally three in number and are situated inthe TG bay.

Drip Pumps: generally two in number of 100% capacity each situatedbeneath the LP heaters.

Turbine Lubricating Oil System: This consists of the Main Oil Pump(MOP), Starting Oil Pump (SOP), AC standby oil pumps and emergency

DC Oil Pump and Jacking Oil Pump (JOP). (one each per unit)

Boiler Feed Pump

This pump is horizontal and of barrel design driven by an Electric Motor

through a hydraulic coupling. All the bearings of pump and motor are forced

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lubricated by a suitable oil lubricating system with adequate protection to trip

the pump if the lubrication oil pressure falls below a preset value.

The high pressure boiler feed pump is a very expensive machine which calls for

a very careful operation and skilled maintenance. Operating staff must be able

to find out the causes of defect at the very beginning, which can be easily

removed without endangering the operator of the power plant and also without

the expensive dismantling of the high pressure feed pump.

Function

The water with the given operating temperature should flow continuously to the

pump under a certain minimum pressure. It passes through the suction branch

into the intake spiral and from there; it is directed to the first impeller. After

leaving the impeller it passes through the distributing passages of the diffuser

and thereby gets a certain pressure rise and at the same time it flows over to the

guide vanes to the inlet of the next impeller. This will repeat from one stage to

the other till it passes through the last impeller and the end diffuser. Thus thefeed water reaching into the discharge space dev